The present invention relates to a thermoplastically processable molding composition of polybutylene terephthalate exhibiting cold impact strength (resistance).
Polybutylene terephthalate is a valuable building material having a series of excellent properties, such as high rigidity, surface hardness, abrasion resistance, high heat distortion stability (high deflection temperature under load), dimensional stability, and rapid processability into complicated and large molded parts.
However, the impact resistance of polybutylene terephthalate, especially at temperatures below 20.degree. C., is inadequate for some areas of application. Consequently, there is a necessity to develop polybutylene terephthalate molding compositions having an improved impact strength.
Numerous suggestions have been advanced for improving the cold impact strength of polybutylene terephthalate molding compositions by incorporating via condensation aliphatic dicarboxylic acids or diol mixtures, or by blending with other polymers, such as modified elastomers and polyolefins.
However, all of the above-described measures suffer from the deficiency that either the increase in cold impact strength is only minor, or a marked improvement in cold impact strength is accompanied by an undesirable impairment of other properties, especially rigidity and deflection temperature under load.
Thus, DOS 2,651,560 describes impact-resistant molding compositions based on a copolyester of terephthalic acid, 1,4-butanediol, and a branched, saturated aliphatic dicarboxylic acid of 7-30 carbon atoms in the molecule. However, copolyesters of this type, as compared with polybutylene terephthalate, exhibit a clearly lower melting point and poorer deflection temperature under load. Cold impact strength is not improved as compared with polybutylene terephthalate.
Blends of polyalkylene terephthalates and elastomeric, segmented copolyesters based on terephthalic acid, alkanediols, and polyalkylene glycol, although satisfying impact strength requirements even under low temperatures, are unsatisfactory because of low rigidity (DOS 2,363,512). Due to the use of the polyalkylene glycols, moreover, the aging and light stability values are unfavorably affected.
Efforts made to raise the cold impact strength of polybutylene terephthalate by additions of acrylonitrile/butadiene/styrene graft polymers (DOS 2,927,576) or of grafted polyolefins (DOS 2,902,468) either resulted in molding compositions showing too great an impairment of the typical properties of polybutylene terephthalate, namely high deflection temperature under load and rigidity, or produced molding compositions without adequate improvement in cold impact strength (DOS 2,927,576).
A similar, unsatisfactory property spectrum for the molding compositions resulted when adding rubber-elastic graft polymers based on acrylate copolymer/acrylonitrile/styrene (DOS 2,444,584) or graft-polymerized products of .alpha.-olefin/vinyl ester copolymers and polymerizable, unsaturated carboxylic acids (esters) (DOS 2,254,002).
The elastomer-addition procedure for rendering polybutylene terephthalate impact resistant which has heretofore been most effective demands functionalization of specific elastomers with maleic anhydride according to methods as described, for example, in DOS 2,401,149. However, during functionalization with maleic anhydride, gel formation occurs because of secondary reactions, the undesirable gel content of the functionalized elastomer being up to 5%.